Process for producing sulfur-containing polymers by polymerizing prepolymers and monomers

ABSTRACT

The process for preparing sulfur-containing polymers, in particular polyarylene sulfides, from at least one sulfide and at least one aromatic dihalogen compound, in a solvent, is carried out in the steps: a) a mixture of aromatic dihalogen compound and sulfide is polymerized, b) aromatic dihalogen compound and sulfide are added to the polymerized mixture and c) the reaction mixture is polymerized further. The sulfur-containing polymers prepared by the process are distinguished by high purity and good mechanical properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is filed pursuant to 35 U.S.C. §371 fromPCT/EP97/02931, filed Jun. 6, 1997, which in turn claim priority toGerman application 196 23 706.8, filed Jun. 14, 1996.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for preparing linear or branchedsulfur-containing polymers, such as polyarylene sulfides, in particularpolyphenylene sulfide (PPS), in which the monomers are added to apolymerized, or partially polymerized, mixture.

2. Description of the Related Art

U.S. Pat. No. 4,910,294 describes a process for preparing PPS. Themonomers used are dihalogenated aromatic hydrocarbons, in particulardichlorobenzene (DCB), and sodium sulfide, which are reacted in ahigh-boiling dipolar aprotic solvent, such as N-methylpyrrolidone (NMP).Substantial amounts of solvent must be employed in order to bring thereaction partners to reaction, since the metal sulfide and the aromaticdihalogen compound reaction components are not miscible with oneanother. At least 3.5 mole of NMP per mole of sulfide are typicallyemployed.

EP-A1-536684 describes the preparation of polyphenylene sulfide in NMPin concentrations higher than 3.5 mol of NMP per mole of sulfide, byadding p-dichlorobenzene to a dehydrated mixture of sodium sulfide andNMP and then polymerizing the mixture. However, a reaction temperatureof 280° C. is required, and reproducibly high molecular weights,characterized by the melt viscosity of the products, are not achieved.

DE-A1-237 110 describes a process for preparing low-molecular-weightpolyarylene sulfide by precharging an alkali metal sulfide in a highboiling polar N-alkyl-2-pyrrolidone and then metering in an aromaticdihalide. Here, the molar ratio of solvent to the sulfide employedreaches at least 3.1. The reaction time is in total 12 h after the startof dihalide addition, at at least 255° C.

EP-B1-215 259 describes a process for preparing polyarylene sulfidesfrom dihaloaromatic compounds and alkali metal sulfide inN-methylcaprolactam in which half of the reaction mixture is prechargedunder gentle reflux. The second half of the mixture, that is includingthe solvent, is then metered in. The minimum ratioachieved of solvent tothe sulfide employed is 3.25. The mean reaction time in the stirredvessel cascade is 12.5 h.

EP 0 374 462 describes the preparation of polyarylene sulfides bycontinuous addition of sulfide to a mixture of dihaloaromatic compoundand polar solvent. The molar ratio of polar solvent to sulfide is fromabout 1.1 to 1.8:1. The reaction times are from about 10 to 15 hours.

If less solvent is employed in the processes of the prior art, molecularweight and polymer yield are poor; at higher solvent concentrations, thespace-time yield is unsatisfactory. To achieve satisfactory space-timeyields, the reaction temperature is frequently raised, but this leads toan increase in side-reactions.

The object is therefore to prepare sulfur-containing polymers, inparticular polyarylene sulfides, over a wide molecular weight range ofmolar mass (e.g. Mw=10,000-200,000 g/mol), with good space-time yield,using the mildest possible reaction conditions, and with the leastpossible contamination by by-products.

BRIEF SUMMARY OF THE INVENTION

It has been found that it is possible to prepare sulfur-containingpolymers, in particular polyarylene sulfides, with high space-timeyield, with a total solvent requirement of less than 300 g of solventper mole of sulfide employed, at reaction temperatures not higher than250° C., and with short reaction times (less than 5 hours), if aprepolymer is firstly formed from aromatic dihalogen compound andsulfide in a polar solvent, and is then converted, by addition ofaromatic dihalogen compound and sulfide, to a polymer of highermolecular weight.

DETAILED DESCRIPTION OF THE INVENTION

The invention therefore relates to a process for preparingsulfur-containing polymers from at least one aromatic dihalogen compoundand at least one sulfide, in a solvent, where a) a mixture of aromaticdihalogen compound and sulfide is polymerized, b) aromatic dihalogencompound and sulfide are added to the polymerized mixture and c) thereaction mixture is polymerized further.

Sulfur-containing polymers are polymers which contain arylene sulfideunits. The arylene constituents of the arylene sulfide units containmono- or polynuclear aromatics or compound aromatics. The aromaticcompounds may also contain heteroatoms. Examples of such aromaticcompounds, which may be substituted or unsubstituted, are benzene,pyridine, biphenyl, naphthalene and phenanthrene. Examples ofsubstituents are C₁-C₆-alkyl, C₁-C₆-alkoxy, carboxyl, amino and sulfonicacid groups. Examples of compound aromatics are biphenyl and aromaticsbonded by ether bridges (arylene ethers).

Preferred sulfur-containing polymers are polyarylene sulfides, inparticular polyphenylene sulfide.

Both inorganic and organic sulfides are suitable as sulfide forpreparing the polymers. Inorganic sulfides are sulfides of alkali metalsand alkaline earth metals, such as lithium sulfide, potassium sulfide,calcium sulfide, and preferably sodium sulfide. The correspondingbisulfides, or hydrogen sulfide, may also be employed, if desiredtogether with alkali metal hydroxides. Suitable organic sulfides aresalt-like sulfides with organic cations. For the purpose of theinvention, organic sulfides are also those organic sulfur compoundswhich liberate sulfide anions or bisulfide anions under the conditionsof the reaction, for example thioacetamide and thio-N-methylpyrrolidone.Sulfides may also be employed with water of crystallization.

Suitable aromatic dihalogen compounds are dihalogenated aromatichydrocarbons, inter alia dihalobenzenes, such as o-, m- andp-dichiorobenzene, substituted dihalobenzenes, such as2,5-dichlorotoluene, 3,5-dichlorobenzoic acid,2,5-dichlorobenzenesulfonic acid and 3,5-dichlorobenzenesulfonic acidand their salts. Dihalonaphthalenes, such as 1,4-dibromonaphthalene anddihalodiphenyl ethers, such as 4,4′-dichlorodiphenyl ether, may,however, also be employed. Mixtures of different arylene dihalides maylikewise be employed. Small amounts (from 0.2 to 5 mol percent based ondihaloaromatic compound) of polyhalogenated aromatic hydrocarbons mayalso be employed in order to obtain branched or crosslinkedsulfur-containing polymers.

Dihaloaromatic compounds and sulfide are also referred to as monomers.

Suitable solvents for preparing the polymer are dipolar aprotic solventsof the amide type, such as dimethylformamide (DMF), dimethylacetamide(DMAc), N-methylcaprolactam and N-alkylated pyrrolidones, such asN-methylpyrrolidone (NMP), or mixtures thereof. NMP is particularlypreferred.

The term prepolymers includes oligomers and polymers containing arylenesulfide units. These products usually have a molar mass, expressed asnumber average molar mass M_(n), in the range from 500 to 20,000 g/mol.They may be linear or branched. It is also possible, by employingsubstituted dihaloaryl compounds (e.g. 2,5-dichlorotoluene) to preparesubstituted prepolymers. Prepolymers with halogen end groups, inparticular chlorine end groups, are preferably prepared.

The preparation of the prepolymer in step a) is generally carried out byreacting sulfide with dihalogen compounds in polar solvents, such asdimethyl sulfoxide, dimethylformamide (DMF), dimethyl acetamide (DMAc),N-alkylated lactams, such as N-alkylcaprolactams, e.g. N-methylcaprolactam (NMC), N-alkylpyrrolidones, e.g. N-methylpyrrolidone (NMP)or mixtures of these, under mild reaction conditions, i.e. attemperatures not above 250° C. The weight of solvent employed per moleof sulfide here is from 150 g to 1000 g, preferably from 250 g to 600 g.If NMP is employed as solvent, this is from 1.5 to 10 mol of NMP permole of sulfide, preferably from 2.5 to 6 mol of NMP per mole ofsulfide. From 0.9 to 1.5 mol of dihaloaromatic compound is employed permole of sulfide. From 1.05 to 1.3 mol is particularly preferable, givinghalogen-terminated prepolymers. The reaction temperatures are from 120to 280° C., preferably from 190 to 250° C. The reaction times in step a)are from 0.1 to 5 h, preferably from 0.2 to 2 h. The conversion toprepolymers, based on sulfide, is from 10 to 98%, preferably from 30 to95%, and particularly preferably from 50 to 80%.

The further addition of the monomers, in step b), to the prepolymerizedmixture comprising solvent, prepolymer, the by-product salt andunconverted monomers, may be carried out in either batch or continuousmanner. The aromatic dihalogen compound is pumped into the reactor asmelt, or with a little solvent, by a metering pump. The sulfides may beadded either in solid form through a valve or in liquid form as a meltedhydrate, using a suitable pump. In a preferred embodiment of theinvention, a homogeneous mixture of sulfide and solvent is added to thereaction mixture with prepolymer. The amount of solvent added with themonomers is adjusted so that there is a total of not more than 300 g ofsolvent per mole of sulfide in the reactor after the addition.

The ratio of the weight of the added monomers m₁ to the weight m₂initially employed for preparing the prepolymer can vary within widelimits. The weight ratio of m₁ to m₂ can be from 0.1 to 10, preferablyfrom 0.5 to 5, and particularly from 1 to 3.

In another preferred embodiment of the invention, the monomers are addedin such a manner that a virtually stationary concentration of themonomers in the reaction mixture is maintained, i.e. the monomerconcentration is held as constant as possible during the reaction. Insuch a stationary state, the amount of monomer added per unit of time isequal to the amount of monomer consumed in the reaction.

If the sulfide is added with water of crystallization, it isadvantageous to remove water from the reaction mixture. This may be donein discrete steps or continuously. Besides water, dihaloaromaticcompound, solvent, and some H₂S are also removed. It has provenadvantageous to remove water from this mixture and to return theremaining components to the reactor.

After the addition of the monomers is complete, the reaction mixture ispolymerized to completion with stirring (step c). The reactionconditions can be varied within wide limits. The reaction temperaturesmay be from 120 to 280° C., preferably from 190 to 250° C. The reactiontimes may be from 20 minutes to 20 hours, preferably from 1 to 3 hours.In a preferred embodiment of the invention, the total reaction time isnot more than 5 hours, at reaction temperatures not higher than 250° C.

All the phases of the preparation of the polymer may be carried outeither batch-wise or continuously. The reaction may, for example, beusing a stirred vessel cascade or in a tubular reactor or using acombination of both of these.

During the reaction, chemically bonded water of crystallization isusually liberated. It can be advantageous for the work-up to remove someor all of the water which is in the reaction mixture, after the reactionis complete. If desired, acids may be added prior to work-up, toneutralize or slightly acidify the reactor contents. Suitable acids are,for example, acetic acid, hydrochloric acid or carbon dioxide.

The polymer is separated off by simple pressure filtration. Othermethods for separating solids from liquids may, however, also beemployed, for example centrifuging or decanting. It is also possible towork up the resultant suspension by flash evaporation or spray drying,drawing off solvents and other low-molecular-weight substances as themain constituents in vapor form and giving the polymer and theby-product salt as substantially dry solids mixture.

In the case of pressure filtration, the filter residue is expedientlywashed with solvent in order to remove residues of mother liquor whichadhere to it. This separation gives polymer and salt as a solid, and themother liquor, which may be reused directly for preparing furtherpolymer. The polymer may be separated from the solid, which may alsofirst be dried, by boiling in water and subsequent filtration.

In a version of the work-up, the hot (from 150 to 240° C.) reactionmixture is filtered under pressure, giving only the salt as residue, thepolymer being redissolved in the filtrate, from which the polymer may beisolated either by spray drying or by crystallization followed byfiltration.

The molecular weights of the polyarylene sulfides may be adjusted todesired values via the stoichiometry of the sulfide and dihaloaromaticcompound monomers. The maximum molecular weight is generally achievedwhen the dihaloaromatic compound is employed in a 2 mol % excess, basedon sulfide.

The novel process is described below using the example of thepreparation of polyphenylene sulfide (PPS), but without being restrictedto this.

Sodium sulfide trihydrate is firstly dissolved by heating in NMP at 180°C. in a titanium autoclave. Some of the water of crystallization is thendistilled off, until an internal temperature of 190° C. has beenreached. The contents of the autoclave are heated further, and at atemperature of from 215 to 220° C. p-dichlorobenzene (DCB) is meteredin. The prepolymer is then formed in a reaction time of from 30 min to 1h at 230° C. After this first reaction phase, both reaction partners aremetered in, continuously or little by little, over a period of about 1h. The molar ratio of the components (sulfide:DCB:NMP) after completedaddition is typically 1:1.05:2.8. During the reaction, the internalpressure of the reactor increases, since chemically bonded water isliberated. It has proven advantageous to control the water content ofthe reaction mixture. One way of achieving this is to limit the internalpressure of the reactor to values between 2 and 6 bar. This may beachieved by partially releasing the gas phase of the reactor into acondenser. After completed addition, stirring of the reaction mixturecontinues for from 1 to 2 hours at from 230 to 235° C. The reactionmixture is cooled and filtered, if desired, after dilution with solvent.The filter residue of PPS and sodium chloride is boiled several timeswith water, filtered, and then dried.

The melting points of the polyphenylene sulfides are from 270 to 305°C., typically from 280 to 295° C. The melt viscosity is in the rangefrom 5000 to 500,000 mPas (centiPoise), preferably from 50,000 to250,000 mPas (centiPoise). The melt viscosity is stable withoutadditives: at 300° C., it changes by less than 10% over a period of 1hour.

The sulfur-containing polymers, such as polyarylene sulfides, inparticular polyphenylene sulfide, prepared by the novel process aredistinguished by high purity and high quality. A particularly noteworthyproperty is that the polymers have virtually no odor and nodiscoloration. The polymers also show good characteristics whensubjected to thermal stress.

The invention thus also relates to sulfur-containing polymers, such aspolyarylene sulfides, prepared by the novel process. Thesulfur-containing polymers prepared according to the invention may beconverted into shaped articles by melt extrusion. Films and fibers withgood mechanical properties may, however, also be produced.

The novel process has a number of advantages:

The amount of solvent employed is at most 300 g of solvent per mole ofsulfide employed, so that a good space-time yield is achieved.

The overall reaction time is less than 5 hours.

The reaction temperature is not above 250° C. and may even besubstantially lower, suppressing side reactions which are among thecauses of toxic impurities in the polymer.

In the overall reaction, the polymer is obtained in a yield of at least90%, typically 95%, based on the amount of sulfide employed.

The following examples illustrate the invention:

EXAMPLE 1

284 g of sodium sulfide trihydrate (about 60%; 2.2 mol) are dissolved in780 g of NMP at 180° C. in a 2 l titanium autoclave. About 130 ml ofcondensate are then distilled off. The autoclave is heated to 230° C.,and, starting at 215° C., 418 g of DCB (1.3×2.2 mol) dissolved in 220 gof warm NMP are added at a rate of 20 ml/min. Prepolymerization thentakes place for 1 h at 230° C. and the autoclave is brought toatmospheric pressure. During this phase of prepolymerization, the molarratio of NMP to sulfide is 4.6. 250 g of sodium sulfide trihydrate (60%;2 mol) are then added to the still hot contents of the autoclave, and amixture of water, DCB and NMP is distilled off until the internaltemperature has reached about 190° C. The mixture is then heated to 230°C., and a further 360 g of DCB in 180 ml of NMP are metered in at a rateof 20 ml/min. Finally, the mixture is polymerized to completion for 1.5h at 230° C., the molar ratio NMP:sulfide now being 2.85. The mixture isdiluted with about 300 ml of NMP, and cooled. The crystalline reactionmixture is filtered, and the residue is boiled in water a number oftimes, filtered off and dried. Yield of PPS: 410 g (95% of theory). Themean molar mass of the polymer is M_(w)=20,000 g/mol.

EXAMPLE 2

230 g of sodium sulfide trihydrate (about 60%; 1.8 mol) are dissolved,with stirring, in 700 g of NMP at 180° C. in a 2 l titanium autoclave.About 95 ml of condensate are then distilled off. The autoclave isheated to 230° C., and, starting at 215° C., 418 g of DCB (1.3×2.2 mol)dissolved in 135 g of warm NMP are added at a rate of 20 ml/min. Theautoclave contents are then held for 1 h at 230° C. During this phase ofprepolymerization, the molar ratio of NMP to sulfide is 4.6. Theautoclave is brought to atmospheric pressure and 192 g of sodium sulfidetrihydrate (60%; 1.5 mol) are then added to the still hot contents ofthe autoclave, and a mixture of water, DCB and NMP is distilled offuntil the internal temperature has reached about 190° C. The mixture isthen heated to 230° C., and a further 227 g of DCB in 102 ml of NMP areadded at a rate of 20 ml/min. Finally, the mixture is polymerized tocompletion for 2.5 h at 230° C., the molar ratio NMP:sulfide now being2.85. The mixture is diluted with about 300 ml of NMP, and cooled. Thecrystalline reaction mixture is filtered, and the residue is boiled inwater a number of times, filtered off and dried. Yield of PPS: 334 g(95% of theory). The melt viscosity at 310° C. and at a shear rate of1000 min⁻¹ is 12 Pas, corresponding to a mean molar mass of M_(w)=20,000g/mol.

Comparative Example.

The following example shows that if the reaction is not carried outaccording to the invention low-molecular-weight polymers are produced inpoor yield.

420 g of sodium sulfide trihydrate (about 60%; 3.2 mol) are dissolved in780 g of NMP at 180° C. in a 2 I titanium autoclave. About 170 ml ofcondensate are then distilled off. The autoclave is heated to 230° C.,and, starting at 215° C., 610 g of DCB (1.3×3.2 mol) dissolved in 320 gof warm NMP are added at a rate of 20 ml/min. Prepolymerization thentakes place for 2 h at 230° C. and the autoclave is brought toatmospheric pressure, the contents are diluted with about 300 ml of NMP,and cooled. The crystalline reaction mixture is filtered, and theresidue is boiled in water a number of times, filtered off and dried.Yield of PPS: 290 g (85% of theory). The mean molar mass of the polymeris M_(w)7000 g/mol.

What is claimed is:
 1. A process for preparing sulfur-containingpolymers from at least one sulfide and at least one aromatic dihalogencompound in a solvent, which comprises: a) polymerizing a mixture ofaromatic dihalogen compound and sulfide; b) adding aromatic dihalogencompound and sulfide to the polymerized mixture; and c) furtherpolymerizing the reaction mixture wherein the molar ratio of solvent tothe sulfide employed does not exceed 3 at the end of the reaction. 2.The process as claimed in claim 1, wherein the conversion based onsulfide in step a) is from 10% to 98% before adding the aromaticdihalogen compound and sulfide in step b) to the polymerized mixture. 3.The process as claimed in claim 1, wherein the conversion based onsulfide in step a) is from 30% to 95% before adding the aromaticdihalogen compound and sulfide to the polymerized mixture.
 4. Theprocess as claimed in claim 1, wherein the conversion based on sulfidein step a) is from 50% to 80% before adding the aromatic dihalogencompound and sulfide to the polymerized mixture.
 5. The process asclaimed in claim 1, wherein the aromatic dihalogen compound and sulfideare added continuously to the polymerized mixture obtained in step a).6. The process as claimed in claim 1, wherein the amount of aromaticdihalogen compound and sulfide added in step b) is 0.1 to 10 times theamount of aromatic dihalogen compound and sulfide used in step a). 7.The process as claimed in claim 1, where the solvent used inN-methylpyrrolidone.
 8. The process as claimed in claim 1, wherein thereaction temperature in the steps a), b) and c) does not exceed 250° C.9. The process as claimed in claim 1, wherein the sum of reaction timesof steps a), b) and c) does not exceed 5 hours.